1. Field of the Invention
The invention relates generally to anti-jamming systems for use with satellite system antennas and, in particular, to anti-jamming systems that are associated with jamming signals that originate along the horizon.
2. Background Information
Global navigation satellite systems (GNSS) provide ranging signals that are utilized in applications that determine global positions of delivery trucks, position information for arriving or departing aircraft, global positions of cellular phones for use by social media and also for emergency 911 purposes, and so forth. As is well known, GNSS antennas receive signals from a plurality of GNSS satellites and associated GNSS receivers determine positions based on the timing of codes and carriers in the received GNSS satellite signals. Increasingly, portable jammers are employed to disrupt particular position calculation operations.
The jammers emit signals at the frequencies of the GNSS satellite signals. The jammer signals that are received by the GNSS antenna interfere with the GNSS satellite signals received by the GNSS antenna and effectively prevent a GNSS receiver from determining an accurate position based on the received GNSS satellite signals. A local jammer may be used, for example, on a delivery truck, to provide jamming signals to the GNSS antenna located on the truck, and thus, prevent the associated GNSS receiver from calculating accurate positions, when the driver wishes to drive the truck on an unauthorized route or at an unauthorized time.
Unfortunately, the signals emitted by a local jammer not only interfere with the GNSS signals received by the co-located GNSS antenna, in the example, the GNSS antenna on the truck, the jammer signals also interfere with the GNSS satellite signals received by nearby GNSS antennas, that is, GNSS antennas that are located within one or two miles of the jammer. Accordingly, as the truck travels along its unauthorized route, the on-board jammer may inadvertently disrupt the operations of various GNSS receivers that provide vital emergency 911 position information, aircraft landing and departing position information, and so forth.
The portable jammers emit jamming signals that have the same frequencies as the GNSS satellite signals and have, at the nearby GNSS antennas, higher power than the GNSS satellite signals which are received after travelling much longer distances through the atmosphere. Thus, the jamming signals overwhelm the GNSS satellite signals at the nearby GNSS antennas, and the GNSS receivers cannot then determine the code and carrier timing needed for position calculations.
The jamming emissions from the jammers of interest can be considered as originating along the horizon. The azimuth angles of the jammer emissions at the nearby GNSS antennas are thus similar to the azimuth angles of signals arriving from low-elevation GNSS satellites that are rising above the horizon and into the sky view of the GNSS antennas. The signals from the low-elevation GNSS satellites may be required for the position calculations, and thus, it is desirable to receive the GNSS satellite signals from the low-elevation GNSS satellites. However, it is not desirable to suffer the adverse effects of interference associated with jamming emissions originating from jammers along the horizon.
What is needed is an anti-jamming mechanism that addresses the adverse effects of the jamming emissions originating along the horizon in the received signals, while at the same time preserving for use in position calculations the received GNSS satellite signals from the higher elevation GNSS satellites as well as the low-elevation satellites that are not in the same directions as the jammers.